Controlling degradation in a reboiler via a hydrophobic coating

ABSTRACT

A method and systems are provided for controlling degradation in a reboiler using a hydrophobic coating. A reboiler is provided that includes a steam shell and a plurality of tubes. The reboiler includes a low surface-energy coating on a surface of the plurality of tubes.

TECHNICAL FIELD

The present disclosure is directed to amine sweetening processes.

BACKGROUND

Gas Plants produce hydrocarbons, such as sales gas, and NGL to supplyindustrial sectors with power. However, natural gases carry acids,namely H2S, CO2, and COS, which hinder the production and separation ofhydrocarbons. For example, the acid gases cause corrosion, andcontaminate product streams. Amines, such as alkanolamine solutions areused as absorbance for the removal of acid gas as they are characterizedwith physical and chemical attributes to effectively absorb acid gas.However, the alkanolamine solutions can be thermally degraded in steamreboilers at high shell temperatures. These degradation products accountas solvent losses and operational costs.

SUMMARY

An exemplary embodiment described herein provides a reboiler thatincludes a steam shell and a plurality of tubes. The reboiler includes alow surface-energy coating on a surface of the plurality of tubes.

Another exemplary embodiment described herein provides a method forcontrolling degradation of a compound in a reboiler. The method includesselecting a low surface-energy coating, applying the low surface-energycoating to a surface of a tube in the reboiler that is in contact withthe compound, and placing the reboiler in service.

Another exemplary embodiment described herein provides an aminestripper. The amine stripper includes a vessel including a rich solventinlet and a lean solvent outlet, and a reboiler. The reboiler includes asteam shell, a plurality of tubes, and a low surface-energy coating on asurface of the plurality of tubes.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of a sweetening system.

FIG. 2 is a schematic drawing of a reboiler used in a sweetening system.

FIG. 3 is a cross-sectional view of the tubes of the reboiler, showing alayer of hydrophobic polymer in contact with the amine solvent.

FIG. 4 is drawing showing the effect of surface tension on contactangle.

FIG. 5 is a method for making a reboiler that lowers degradation ofcompounds in contact with the reboiler.

DETAILED DESCRIPTION

Techniques are provided herein to reduce shell temperature by increasingnucleate heat transfer coefficient. As described in examples herein,hydrophobic materials are deposited along tubing surfaces that are indirect contact with alkanolamine solvents. The hydrophobic coating layermay minimize degradation of the alkanolamine solvents, and thus minimizesolvent losses, reduce steam consumption, and enhance process integrity.

FIG. 1 is a schematic diagram of a sweetening system 100. The sweeteningsystem 100 includes an amine contactor 102, a flash drum 104, an aminestripper 106, and a reboiler 108 on the amine stripper 106. Furtherunits that can be used in the system include an amine circulation pump110, an amine cooler 112, and a reflux chiller 114 on the amine stripper106. As used herein, amine includes alkanolamine solvents, or othertypes of amine solvents that may be used for the absorption of acidgases in a sweetening system 100.

The amine contactor 102 is a counter-flow gas-liquid contactor that canbe referred as an absorber, treater, or scrubber. The amine contactorgenerally includes internal components, such as trays or packing, toincrease gas-liquid contact.

The flash drum 104 operates at a lower pressure than the amine contactor102 and allows light hydrocarbons to flash or evaporate from the aminesolvent. The flash drum 104 is sized for liquid surge, liquid holdup,and residence time for vapor to separate from the liquid amine solvent.In some embodiments, the flash drum 104 is equipped with a flash drumtower 116. The flash drum tower 116 can remove acid gas such as hydrogensulfide, which can be present in the vapor stream 118 separated from theamine solvent, before the vapor stream 118 is sent to another downstreamprocess or end user.

The amine stripper 106 is a vessel, which can also be referred to as aregenerator. The amine stripper 106 comprises internal components, forexample, trays or packing, and effectively serves as a distillationtower to boil off acid gas to regenerate the amine solvent. In someembodiments, the amine stripper 106 includes a reflux chiller 114 tocool a flow from the top of the amine stripper 106, and returncondensate to the amine stripper 106 as a reflux stream 120. Thedistinction between acid gas and sour gas is that sour gas is mostlyhydrocarbons with some acidic gas content, and acid gas contains littleto no hydrocarbons.

The circulation pump 110 pressurizes the regenerated amine, e.g., thelean solvent stream 122, to recycle back to the amine contactor 102 as apressurized stream 124. The circulation pump 110 can comprise a singlepump or multiple pumps in parallel or in series. The circulation pump110 can be sized to accommodate upset scenarios, which require muchhigher flow rates than is normally required by the sweetening system100.

The amine cooler 112 brings the temperature of the pressurized stream124 down before the cooled solvent stream 126 is recycled back to theamine contactor 102. The lower temperature of the solvent stream 126increases the efficiency of cleaning the sour gas stream 128 that entersthe amine contactor 102. The amine cooler 112 can be a shell-and-tubeheat exchanger, an air cooler, or a combination of multiples of both.

Gas sweetening units can optionally comprise auxiliary and variantequipment such as additional heat exchangers and vessels that have notbeen described above, but a majority of gas sweetening units across theworld implement some variation or combination of the major equipmentoutlined.

The sweetening system 100 can operate at a variety of operatingtemperatures and pressures. In some embodiments, sour gas at atemperature of between about 70 and about 130° F. enters the bottom ofthe amine contactor 102 via sour gas stream 128, as the amine solventstream 126 enters from the top of the amine contactor 102 at atemperature of between about 80 and about 140° F. The amine solventstream 126 that enters the amine contactor 102 is at least about 10° F.hotter than the sour gas stream 128 that enters the amine contactor 102.As the amine solvent contacts the sour gas, the solvent absorbs thesulfur compounds, carbon dioxide, and other contaminants from the sourgas, by chemical and physical binding.

Once the solvent has passed through amine contactor 102, a rich solventstream 130 exiting the amine contactor 102 is considered to be in a“rich” state, also referred as “rich solvent”, because the solventcontains the acid gases removed from the sour gas. A sweetened gasstream 132 exits from the top of the amine contactor 102. The sweetenedgas can contain about 5 ppm to about 60 ppm hydrogen sulfide. Thesweetened gas is sent downstream for sale or further processing. Therich solvent stream 130 is sent to the flash drum 104, which operates ata pressure between about atmospheric pressure to about 90 psig, whereany flashed vapor travels up the flash drum tower 116 and exits as thevapor stream 118, where the flashed vapor can then be utilized as fuel,vented, flared, or a combination of these.

The rich solvent stream 134 from the flash drum 104 is sent to the aminestripper 106 with a top operating pressure between 5 and about 17 psig.The hydrogen sulfide and carbon dioxide is boiled off via heat input tothe bottom of stripper 106 by the reboiler 108. The reboiler operates ata temperature range of between about 230 to about 270° F. in order toregenerate the amine solvent. The regenerated solvent is then consideredto be in a “lean” state, also referred as “lean solvent,” that is onceagain suitable to be used for cleaning additional sour gas.

A mixed gas stream 136, comprising some hydrocarbons, hydrogen sulfide,and carbon dioxide exits the top of the stripper 106. The mixed gasstream 136 passes through the reflux chiller 114, and the reflux stream120, including hydrocarbons condensed in the reflux chiller 114, isreturned to the stripper 106. An acid gas stream 138, including hydrogensulfide and carbon dioxide, exits the reflux chiller 114 to be passed todownstream processes, or waste.

The lean solvent stream 122 that is pumped out of the bottom of thestripper 106 by the circulation pump 110 is cooled in an amine cooler112 to about 80 to about 140° F. before re-entering the amine contactor102 to be used again to clean additional sour gas. The transport ofvapor and liquid within, to, and from the sweetening system 100 can beachieved using various piping, pump, and valve configurations.

In this example, if the reboiler 108 is considered a single-passreboiler, in which the feed stream 140 into the reboiler 108 is takenfrom above one of the plates 142 in the stripper 106. The heated returnstream 144 is fed to the bottoms 146 of the stripper 106. The reboiler108 is described further with respect to FIG. 2.

FIG. 2 is a schematic drawing of a reboiler 108 used in a sweeteningsystem. Like numbered items are as described with respect to FIG. 1. Asshown in FIG. 1, the acid gases flashed off the amine solvent, oralkanolamine solution, by a stripping vapor, mostly steam, generated inthe reboiler 108. In the reboiler 108, the feed stream 140 enters abottom chamber 202. From the bottom chamber 202, the amine solvent flowsupwards through the tubes 204 of the reboiler 108. Steam is introducedinto the reboiler through a steam inlet line 206 and flows through thespace 208 around the tubes 204, heating the tubes 204 and the aminesolvent in the tubes 204. The steam then exits the reboiler through asteam outlet line 210. Although the steam inlet line 206 and the steamoutlet line 210 may be reversed, generally the steam is introduced atthe top of the heated vessel to force condensate from the steam out.

As described further with respect to FIG. 3, the amine solvent in thetubes 204 is heated to form a two-phase flow that provides the motiveforce to flow the amine solvent into the top chamber 212 of the reboiler108. The pressure than forces the amine solvent from the top chamber 212back to the amine stripper 106 (FIG. 1) through the heated return stream144.

FIG. 3 is a cross-sectional view of the tubes 204 of the reboiler 108,showing a layer 302 of hydrophobic polymer in contact with the aminesolvent 304. Like numbered items are as described with respect to FIGS.1 and 2. As described with respect to FIG. 2, the amine solvent 304enters a bottom chamber 202 of the reboiler 108, and flows upwardsthrough the tubes 204. Fresh steam 306 is introduced to the interior ofthe reboiler 108 and the space 208 around the tubes 204. Circulatingsteam 308 flows around the tubes 204 transferring heat from thecirculating steam 308 to the tubes 204 and the amine solvent 304 flowingthrough the tubes 204. Outlet steam 310, including condensate and lowertemperature steam, then exits the reboiler 108.

Generally, reboilers used with stripper columns are classified asonce-through thermosiphon reboilers. The fluid flow upwards of the aminesolvent 304 is created by buoyancy forces evolved from a densitygradient induced by temperature differences. The density gradient inreboilers is often a combination of two effects, the lower density ofhotter fluids and the presence of two-phase flow in the tubes 204 of thereboiler 108 Due to the pumpless nature of flow in thermosiphonreboilers, the fluid circulation, and heat transfer are coupled.

The heat transfer mechanism is quantitatively expressed by the heattransfer coefficient. Equation (1) shows the relationship between heatflux and heat transfer coefficient, where “q” is the heat flux in W/m²(watts per square meter), “h” is the heat transfer coefficient in W/m²-K(watts per square meter per degree kelvin), and “ΔT” is the temperaturedifferential in Kelvin.

q=h*ΔT   (1)

When the amine solvent 304 enters the two-phase regime, for example, dothe boiling of water or gases dissolved in the amine solvent 304, a newheat transfer mechanism occurs to enhance the boiling phenomenon. Thismechanism is known as nucleate boiling. Nucleate boiling is enhanced byhigher surface temperature, higher surface curvature, or lower energysurface. In operation, only surface temperature can be manipulated tocontrol the degree of boiling.

However, the high surface temperature significantly increases thedegradation rate of alkanolamine, because degradation kinetics areexponentially proportional to temperature. Equation (2) shows the rateof diglycolamine degradation products in reboiler tubes, where “X_(o)”is converted mol fraction at the outlet in mol/mol, “X_(i)” is convertedmol fraction at the inlet in mol/mol, “k_(T)” is rate constant of thereaction in 1/hr, “ρ” is the density of the fluid in kg/m³, “l” is thelength of tube in meters, and “G” is the mass flux in kg/m²-hr.

$\begin{matrix}{X_{o} = {1 - {\left( {1 - X_{i}} \right)*{\exp\left( {{- k_{T}}*\frac{\rho l}{g}} \right)}}}} & (2)\end{matrix}$

Equation (3) show the pseudo first-order kinetics of alkanolaminethermal degradation. “Ea” is the activation energy in J/mol, “R” is theideal gas constant in J/mol-K, and “T” is the temperature in Kelvin.Both activation energy and k_(165° C.), which is the reaction rateconstant at 165° C., were experimentally determined.

$\begin{matrix}{{\ln\left( \frac{k_{T}}{k_{165{^\circ}{C.}}} \right)} = {\frac{E_{a}}{R}\left( {\frac{1}{\left( {{165} + 273.15} \right)} - \frac{1}{T}} \right)}} & \left( {A\text{.3}} \right)\end{matrix}$

As such, it is preferable to either increase surface curvature or reducesurface energy, e.g., surface tension, to effectively reduce the surfacetemperature and the degradation rate. A hydrophobic coating reducessurface energy by increasing the degree of contact angle between vaporbubble and surface.

FIG. 4 is drawing showing the effect of surface tension on contactangle. As described herein, lowering the surface energy, for example, byapplying the layer 302 of the hydrophilic polymer, lowers the surfacetension. Like numbered items are as described with respect to theprevious figures. The flow control of steam to the reboiler 108 iscontrolled by the overhead temperature of the stripper 106, e.g., thetemperature at the top of the stripper 106. Maintaining the overheadtemperature is performed by holding the mass flowrate of stripper vapor,for example, from flashing gas in the rich solvent stream 134 enteringthe stripper 106 and vapor generated in the reboiler 108, to beconstant. Accordingly, the flow of steam into the steam shell to holdthe heat flux from the steam shell to tube fluid in the reboiler 108constant. As described in Equation (1), increasing heat transfercoefficient reduces the temperature differential.

Another factor that has been introduced is “Ψ”, or the Takata factor,which details the proportionality of heat transfer coefficient withcontact angle. This relationship is expressed in Equation (4). Thedependence of Takata factor on contact angle is shown in Equation (5),while Young's module shows the contact angle as a function of surfacetensions. In these equations, and as shown in FIG. 4, “θ” is the contactangle, “σ_(SV)” is surface tension between solid surface and vapor,“σ_(Sl)” is surface tension between solid surface and liquid, and“σ_(lv)” is surface tension between liquid and vapor.

$\begin{matrix}{h_{2} = {h_{1}*\frac{\Psi\left( \theta_{2} \right)}{\Psi\left( \theta_{1} \right)}}} & (4)\end{matrix}$ $\begin{matrix}{{\Psi(\theta)} = {{\tan\theta^{1/6}} + {{0.2}5*\tan\theta^{{- 1}/2}}}} & (5)\end{matrix}$ $\begin{matrix}{{\cos(\theta)} = \frac{\sigma_{sv} - \sigma_{sl}}{\sigma_{lv}}} & (6)\end{matrix}$

One example of a hydrophobic coating that may be used as layer 302 isTeflon. Correlations have been developed to theoretically calculatesurface tensions with stainless steel and Teflon. The contact anglesestimated for stainless steel and Teflon substrate were 12.3° and 76.5°,respectively. According to Equations (4) and (5), Teflon coating willenhance heat transfer coefficient by 6 to 8%. This ultimately willreduce the rate of thermal degradation rate of alkanolamine at film andwall positions by 12% and 21%, respectively.

Other hydrophobic polymers may be used as the layer 302, include, forexample, ultrahigh molecular weight polyethylene (UHMPE), polyphenylenesulfide (PPS), or polyphenylene oxide (PPO), among many others. Inaddition to the surface energy of the coating, the ability of thecoating to hold up under the high temperatures and chemical environmentinvolved in the reboiler is another factor in choosing the material forthe hydrophobic coating. For example, a polymer with a high temperatureresistance and resistance to the alkanolamine solvent may provide abetter choice than a polymer with a lower surface energy.

Other types of materials may be used as the hydrophobic coating inembodiments. For example, in various embodiments, the hydrophobiccoating may be a hydrophobic surface treatment of an interior surface ofthe tubes 204. Other materials that may be used as hydrophobic coatingsmay include carbon nanotubes, among others.

FIG. 5 is a method 500 for making a reboiler that lowers degradation ofcompounds in contact with the reboiler. The method begins at block 502,with the selection of a low surface-energy coating material. The coatingmaterial may be selected as discussed with respect to FIG. 4, forexample, to balance temperature resistance, chemical resistance, and lowsurface energy. In some embodiments, the coating material is a surfacetreatment to lower the surface energy of the system, such as theapplication of carbon nanotubes among others.

At block 504, the coating is applied to the tubes in the reboiler. Thecoating may be applied to the tubes of the reboiler using a powdercoating process, after which the coating is melted to the surface of thetubes. In some embodiments, the powder coating is conducted by anelectrostatic process. In some embodiments, the coating is applied tothe tubes as a solution of a polymer in a solvent. After the coating isapplied, the solvent is removed by drying. Other techniques for applyingthe coating, such as growing carbon nanotubes from the surface of thetubes, may be used. After the tubes are coated, the reboiler may beassembled. In some embodiments, the tube assembly, for example,including the tubes welded to an upper distribution plate and a lowerdistribution plate, is assembled first, after which the coating isperformed.

In addition to the alkanolamine described herein, reboilers used inother types of systems may benefit from the coating described. In thesesystems, the low surface-energy coating material may be selected tolower the surface energy for other types of chemical systems. In someembodiments, a hydrophilic coating that has a low surface energy towetting from a hydrocarbon, and is resistant to other organic chemicals,such as hydrocarbons, is selected for reboilers in refineries andchemical plants.

At block 506, the modified reboiler if placed in service in the plantchosen. In some embodiments, this is in an amine stripper, as describedherein. In other embodiments, this is in a distillation tower in arefinery or chemical plant.

Embodiments

An exemplary embodiment described herein provides a reboiler thatincludes a steam shell and a plurality of tubes. The reboiler includes alow surface-energy coating on a surface of the plurality of tubes.

In an aspect, the reboiler includes a once-through thermosiphon design.

In an aspect, the reboiler includes a steam inlet to the steam shell,and a steam outlet from the steam shell. The steam inlet is disposed atthe top of the steam shell and the steam outlet is disposed at thebottom of the steam shell.

In an aspect, the reboiler includes an organic compound flowing upwardsthrough the plurality of tubes. In an aspect, the organic compoundincludes alkanolamine. In an aspect, the low surface-energy coating ison the surface of the plurality of tubes that is in contact with theorganic compound.

In an aspect, the low surface-energy coating includes a hydrophobicpolymer. In an aspect, the hydrophobic polymer includespolytetrafluoroethylene. In an aspect, the hydrophobic polymer includespolyphenylene oxide, polyphenylene sulfide, or ultrahigh molecularweight polyethylene, or any combinations thereof. In an aspect, the lowsurface-energy coating includes carbon nanotubes. In an aspect, the lowsurface-energy coating includes a texturing of the surface.

In an aspect, the reboiler includes an alkanolamine inlet coupled to anamine stripper, and an alkanolamine outlet coupled to the aminestripper.

Another exemplary embodiment described herein provides a method forcontrolling degradation of a compound in a reboiler. The method includesselecting a low surface-energy coating, applying the low surface-energycoating to a surface of a tube in the reboiler that is in contact withthe compound, and placing the reboiler in service.

In an aspect, the low surface-energy coating is selected by the surfaceenergy to be hydrophobic. In an aspect, the low surface-energy coatingis selected by resistance to the compound in contact with the lowsurface-energy coating. In an aspect, the low surface-energy coating isselected by resistance to an operating temperature of the reboiler.

In an aspect, the low surface-energy coating is applied to the surfaceof the tube as a powder coating, which is then fused onto the surface ofthe tube. In an aspect, the low surface-energy coating is sprayed ontothe surface of the tube in a solution, and a solvent in the solution isthen evaporated.

In an aspect, the reboiler is placed into service by being fluidicallycoupled to an amine stripper. In an aspect, the reboiler is placed intoservice by being fluidically coupled to a distillation column in arefinery or chemical plant.

Another exemplary embodiment described herein provides an aminestripper. The amine stripper includes a vessel including a rich solventinlet and a lean solvent outlet, and a reboiler. The reboiler includes asteam shell, a plurality of tubes, and a low surface-energy coating on asurface of the plurality of tubes.

In an aspect, the amine stripper includes a reflux chiller, wherein thereflux chiller is configured to provide a reflux flow back to thevessel. In an aspect, the amine stripper includes a fluidic couplingabove a plate in the amine stripper to an inlet at the bottom of thereboiler, and a fluidic coupling from an outlet at the top of thereboiler to the amine stripper below the plate.

In an aspect, the amine stripper includes the low surface-energy coatingon the surface of the plurality of tubes that is in contact with thesolvent. In an aspect, the solvent includes alkanolamine.

Other implementations are also within the scope of the following claims.

What is claimed is:
 1. A reboiler, comprising: a steam shell; aplurality of tubes; and a low surface-energy coating on a surface of theplurality of tubes.
 2. The reboiler of claim 1, comprising aonce-through thermosiphon design.
 3. The reboiler of claim 1,comprising: a steam inlet to the steam shell; and a steam outlet fromthe steam shell, wherein the steam inlet is disposed at the top of thesteam shell and the steam outlet is disposed at the bottom of the steamshell.
 4. The reboiler of claim 1, comprising an organic compoundflowing upwards through the plurality of tubes.
 5. The reboiler of claim4, wherein the organic compound comprises alkanolamine.
 6. The reboilerof claim 4, comprising the low surface-energy coating on the surface ofthe plurality of tubes that is in contact with the organic compound. 7.The reboiler of claim 1, wherein the low surface-energy coatingcomprises a hydrophobic polymer.
 8. The reboiler of claim 7, wherein thehydrophobic polymer comprises polytetrafluoroethylene.
 9. The reboilerof claim 7, wherein the hydrophobic polymer comprises polyphenyleneoxide, polyphenylene sulfide, or ultrahigh molecular weightpolyethylene, or any combinations thereof.
 10. The reboiler of claim 1,wherein the low surface-energy coating comprises carbon nanotubes. 11.The reboiler of claim 1, wherein the low surface-energy coatingcomprises a texturing of the surface.
 12. The reboiler of claim 1,comprising: an alkanolamine inlet coupled to an amine stripper; and analkanolamine outlet coupled to the amine stripper.
 13. A method forcontrolling degradation of a compound in a reboiler, comprising:selecting a low surface-energy coating; applying the low surface-energycoating to a surface of a tube in the reboiler that is in contact withthe compound; and placing the reboiler in service.
 14. The method ofclaim 13, wherein the low surface-energy coating is selected by thesurface energy to be hydrophobic.
 15. The method of claim 13, whereinthe low surface-energy coating is selected by resistance to the compoundin contact with the low surface-energy coating.
 16. The method of claim13, wherein the low surface-energy coating is selected by resistance toan operating temperature of the reboiler.
 17. The method of claim 13,wherein the low surface-energy coating is applied to the surface of thetube as a powder coating, which is then fused onto the surface of thetube.
 18. The method of claim 13, wherein the low surface-energy coatingis sprayed onto the surface of the tube in a solution, and a solvent inthe solution is then evaporated.
 19. The method of claim 13, wherein thereboiler is placed into service by being fluidically coupled to an aminestripper.
 20. The method of claim 13, wherein the reboiler is placedinto service by being fluidically coupled to a distillation column in arefinery or chemical plant.
 21. An amine stripper, comprising: a vesselcomprising a rich solvent inlet and a lean solvent outlet; and areboiler comprising: a steam shell; a plurality of tubes; and a lowsurface-energy coating on a surface of the plurality of tubes.
 22. Theamine stripper of claim 21, comprising a reflux chiller, wherein thereflux chiller is configured to provide a reflux flow back to thevessel.
 23. The amine stripper of claim 21, comprising a fluidiccoupling above a plate in the amine stripper to an inlet at the bottomof the reboiler, and a fluidic coupling from an outlet at the top of thereboiler to the amine stripper below the plate.
 24. The amine stripperof claim 21, comprising the low surface-energy coating on the surface ofthe plurality of tubes that is in contact with the solvent.
 25. Theamine stripper of claim 21, wherein the solvent comprises alkanolamine.